The present invention relates to purifying method of fluoride glass which is used for an optical fiber, in particular, relates to purifying method of ZrF.sub.4 based and/or H.sub.f F.sub.4 based fluoride glass.
It is theoretically analyzed that a fluoride glass optical fiber, in particular, ZrF.sub.4 based and/or H.sub.f F.sub.4 based fluoride glass optical fiber allows infrared transmission (2-4 .mu.m wavelength) with an extremely small loss of less than 0.1 dB/km, which is less than that of a conventional silica based optical fiber. Therefore, a fluoride glass optical fiber is very promising for a future optical fiber.
The problem to be solved in a conventional fluoride optical fiber is the light absorption around 3 .mu.m wavelength by molecule vibration of hydroxyl group (OH) which is included in fluoride glass.
Conventionally, a fluoride glass has been dehydrated as the same manner as that of a silica based glass. That is to say, a fluoride glass is first melted in atmosphere of halogen gas (for instance, Cl.sub.2, CCl.sub.4, SF.sub.6 et al) so that the dehydration is accomplished by chemical reaction between hydroxyl group included in fluoride glass and halogen gas. Although that prior dehydration process is also effective to dehydrate fluoride glass, it has the serious disadvantage that the transmission loss of the glass increases considerably by scattering. Accordingly, the chemical dehydration is not used at present, instead, a glass is manufactured in dried gas atmosphere so that a glass does not include water in producing process.
The concentration of hydroxyl group (OH) must be less than 1 ppm, or less for a small loss optical fiber. However, a prior producing method which manufactures glass in dried atmosphere can not exclude water and/or hydroxyl group to that value. Further, as mentioned above, a prior chemical process has the disavantage that a scattering loss is increased.
Thus, no effective dehydration process for fluoride glass has not been proposed.